GENOME

In modern molecular biology and genetics, the genome is the genetic material of an organism. It is
encoded either in DNA or, for many types of viruses, in RNA. The genome includes both the genes and
the non-coding sequences of the DNA/RNA.
Some organisms have multiple copies of chromosomes: diploid, triploid, tetraploid and so on. In classical
genetics, in a sexually reproducing organism (typically eukarya) the gamete has half the number of
chromosomes of the somatic cell and the genome is a full set of chromosomes in a diploid cell. The
halving of the genetic material in gametes is accomplished by the segregation of homologous
chromosomes during meiosis.[5] In haploid organisms, including cells of bacteria, archaea, and in
organelles including mitochondria and chloroplasts, or viruses, that similarly contain genes, the single or
set of circular or linear chains of DNA (or RNA for some viruses), likewise constitute the genome. The
term genome can be applied specifically to mean what is stored on a complete set of nuclear DNA (i.e.,
the "nuclear genome") but can also be applied to what is stored within organelles that contain their own
DNA, as with the "mitochondrial genome" or the "chloroplast genome". Additionally, the genome can
comprise non-chromosomal genetic elements such as viruses, plasmids, and transposable elements.[6]

When people say that the genome of a sexually reproducing species has been "sequenced", typically
they are referring to a determination of the sequences of one set of autosomes and one of each type of
sex chromosome, which together represent both of the possible sexes. Even in species that exist in only
one sex, what is described as a "genome sequence" may be a composite read from the chromosomes of
various individuals. Colloquially, the phrase "genetic makeup" is sometimes used to signify the genome
of a particular individual or organism.[citation needed] The study of the global properties of genomes of
related organisms is usually referred to as genomics, which distinguishes it from genetics which
generally studies the properties of single genes or groups of genes.

Both the number of base pairs and the number of genes vary widely from one species to another, and
there is only a rough correlation between the two (an observation known as the C-value paradox). At
present, the highest known number of genes is around 60,000, for the protozoan causing trichomoniasis
(see List of sequenced eukaryotic genomes), almost three times as many as in the human genome.

Composition
Genome composition is used to describe the make up of contents of a haploid genome, which should
include genome size, proportions of non-repetitive DNA and repetitive DNA in details. By comparing the
genome compositions between genomes, scientists can better understand the evolutionary history of a
given genome.

When talking about genome composition, one should distinguish between prokaryotes and eukaryotes
as the big differences on contents structure they have. In prokaryotes, most of the genome (85-90%) is
non-repetitive DNA, which means coding DNA mainly forms it, while non-coding regions only take a
small part.[13] On the contrary, eukaryotes have the feature of exon-intron organization of protein
coding genes; the variation of repetitive DNA content in eukaryotes is also extremely high. When refer
to mammalians and plants, the major part of genome is composed by repetitive DNA.[14]

Most biological entities that are more complex than a virus sometimes or always carry additional genetic
material besides that which resides in their chromosomes. In some contexts, such as sequencing the
genome of a pathogenic microbe, "genome" is meant to include information stored on this auxiliary
material, which is carried in plasmids. In such circumstances then, "genome" describes all of the genes
and information on non-coding DNA that have the potential to be present.

In eukaryotes such as plants, protozoa and animals, however, "genome" carries the typical connotation
of only information on chromosomal DNA. So although these organisms contain chloroplasts or
mitochondria that have their own DNA, the genetic information contained by DNA within these
organelles is not considered part of the genome. In fact, mitochondria are sometimes said to have their
own genome often referred to as the "mitochondrial genome". The DNA found within the chloroplast
may be referred to as the "plastome".

Genome size[edit]

Log-log plot of the total number of annotated proteins in genomes submitted to GenBank as a function
of genome size.[15]
Genome size is the total number of DNA base pairs in one copy of a haploid genome. The genome size is
positively correlated with the morphological complexity among prokaryotes and lower eukaryotes;
however, after mollusks and all the other higher eukaryotes above, this correlation is no longer
effective.[14][16] This phenomenon also indicates the mighty influence coming from repetitive DNA act
on the genomes.

Since genomes are very complex, one research strategy is to reduce the number of genes in a genome
to the bare minimum and still have the organism in question survive. There is experimental work being
done on minimal genomes for single cell organisms as well as minimal genomes for multi-cellular
organisms (see Developmental biology). The work is both in vivo and in silico.